Abstract Sustained attention (SA), the ability to maintain focus on a task over prolonged periods of time, is a fundamental cognitive process that directly impacts other cognitive functions, such as learning from a lecture or from reading. Fluctuations of SA are commonly observed, but their causes seem to be heterogenous, and the ways in which they influence ongoing information processing and subsequent memory are not well understood. Some researchers have proposed that SA fluctuations result from a growing tendency with increasing task duration towards mind-wandering (attention drifting towards internal thoughts or distractors), due to failure of executive control processes. Similarly, failures of executive control also tend to allow the capture of attention by external distractors. The main goal of this proposal is to investigate how the direction of attention (external vs. internal) and the level of control interact to determine fluctuations of SA, and how they affect current learning. In Aim 1, the applicant will investigate two types of attention lapses (externally distracted and mind-wandering) that can occur during SA, including the rates at which they occur over time and how they each influence subsequent memory. The applicant will implement a novel paradigm in which task-relevant object images are presented on top of much larger distracting face images, and thought probes are used to assess the individuals? attention state as either on-task, externally distracted, or mind-wandering, while pupillary responses are recorded. The main hypothesis is that rates of external distraction and mind-wandering will increase over time as the maintenance of executive control diminishes. In addition, the applicant will investigate the distinct influence of each individual attention-lapse state on subsequent memory. This study will be the first to relate internal vs external lapses of attention during SA to memory performance using pupillometry measures. In Aims 2a and 2b, fMRI methodology, along with in-scanner pupillometry recordings, will be used to examine the neural underpinnings of external distraction vs mind-wandering during SA and to uncover the distinct neural mechanisms by which these attention states influence memory. This Aim will include novel application of attention-state classifier analyses of the fMRI data and how these states relate to the behavior and pupillometry measures, as well as innovative dynamic functional connectivity approaches to investigate the interplay within and between networks that gives rise to SA fluctuations and their effects on memory. This study will offer insights into the roles of the dorsal attention network, the default-mode network, and the fronto-parietal control network in SA, as well as the mnemonic costs of external distraction vs mind-wandering, to gain a deeper understanding of the neural mechanisms through which SA impacts memory. This project will substantially increase our understanding of the different ways in which SA can be disrupted and the unique influences that lapses in SA have on subsequent memory. Thus, the results of these studies may in turn lead to improvement in identifying SA deficits in both the normal population and in various clinical disorders, and suggest possible approaches as to how they may be addressed.